Accidently deleted a theme
As @MrZak suggested, you can get the files from online resources.


Go to the "interface_theme" folder in the Blender Source Code
Copy the contents of the XML file for the theme you want
Go to the folder in which blender is installed(usually located in program files)
Navigate to "2.79\scripts\presets\interface_theme"
Create a new XML file and paste the previously copied item in it